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Strangeness as a deconfinement signature

In the deconfined phase, partonic reactions change the number of strange quarks:

$$g+g \leftrightarrow s+\bar{s},$$ (34)

$$q+\bar{q} \leftrightarrow s+\bar{s},$$ (35)

where $q$ and $\bar{q}$ denote quarks and antiquarks, and $g$ - gluons. In the hadronic gas, the relevant processes are

$$\pi + \pi \leftrightarrow K + \bar{K}$$ (36)

$$\pi + N \leftrightarrow Y + K$$ (37)

$$N+N \leftrightarrow N+Y+\bar{K}$$ (38)

$$N+N \leftrightarrow N+N+\bar{K}+K$$ (39)

Here $Y$ stands for a $\Lambda$ or $\Sigma$ hyperon, and kaons are $K = \bar{q}s$ ($K^-$, $\bar{K^0}$) and $\bar{K} = q\bar{s}$ ($K^+$, $K^0$). Reaction 5.4 may proceed through intermediate stages involving heavy resonance formation, their interaction with the medium, excitation and decay through the $Y + K$ channel. In that case, the right-hand side may be not the only product.

The first proposal of a strangeness-based QGP signature was made by Rafelski and Hagedorn [20] in 1981. It did not involve a detailed analysis of hadrochemical kinetics, but was based on a statistical model approach advocated by Hagedorn [45]. Assuming equilibration of strangeness in QGP, they estimated that for the ratio of baryochemical potential to temperature $\mu/T \sim 6-7$, one could expect ratio $\bar{s}/\bar{q} \sim 5$. The enhancement was expected to be stronger for higher baryochemical potential since that would exclusively suppress $q\bar{q}$ (and not $s\bar{s}$) production.

In a subsequent work, Rafelski and Müller [21] used lowest order perturbative QCD calculations to obtain the production rate of $s\bar{s}$ pairs in reactions with quarks and gluons in the initial state. They found that the predominant fraction of strangeness is produced in gluon-gluon reactions, and that consequently the strangeness per baryon number in QGP saturates over the time of the order of 10 fm/$c$.

Besides that, it was pointed out [20] that ``some of the numerous $\bar{s}$ may, instead of being bound in a $q\bar{s}$ kaon, enter into a $\bar{q}\bar{q}\bar{s}$ antibaryon and, in particular, a $\bar{\Lambda}$ or $\bar{\Sigma^0}$.'' In hadronic gas, such particles can be created only in direct pair production reactions, which is kinematically suppressed by a high threshold. This makes strange antibaryons the most characteristic strangeness-related QGP signature. However, it was also emphasized [46] that $K^+$ abundance deserves attention because ``about half of the $\bar{s}$ quarks from the plasma will be used in making $K^+$ mesons, the other half contributing to $K^0 \pm \bar{K^0}$ states, and a smaller, and for this consideration, insignificant number of $\bar{s}$ quarks being contained in the antistrange baryons; $\bar{s}\bar{s}\bar{s}$, $\bar{s}\bar{s}\bar{q}$, $\bar{s}\bar{q}\bar{q}$, or $s\bar{s}$ mesons, as it is self-evident that such states have a much smaller chance of emerging from a baryon-rich plasma than does a $\bar{s}q$ meson.'' On the contrary, kaons with an $s$ quark ( $K^-=s\bar{u}$, $\bar{K^0}=s\bar{d}$), due to their large strangeness exchange cross-section in hadronic gas, represent mainly the post-hadronization stage in the evolution of the system. Because $K^0$ and $\bar{K^0}$ are mixed in the decay eigenstates $K_S$ and $K_L$ (so that no distinction can be made between the $s$ and $\bar{s}$ meson), neutral kaons are less interesting than $K^+$ from the QGP signal point of view [46].